Common types of filters currently known in the art have not been particularly suited for these desired improvements. LC filters can be low cost and tunable, but have high loss and become too small to be practical at microwave frequencies. Surface acoustic wave (SAW) filters work well at microwave frequencies, but they are not tunable.
Planar filters and cavity filters appear to provide the best option for narrowband filters at microwave frequencies. They have better performance and lower loss when the resonators are surrounded by air instead of lossy dielectric material. A conventional, air dielectric, filter of these types would be constructed using an expensive machined-metal housing. This invention constructs the metalized features of a microwave filter an air cavity embedded in the interior layers of a multilayer printed circuit board. Also, microwave filters may be tuned to frequency by the addition of reactance to the circuit. In a conventional, machined-housing filter it is difficult to attach tuning elements to the ends of the resonators because the resonators are suspended in air. In this invention, as the construction uses printed circuit fabrication, tuning elements are easily added external to the air cavity and connected by metalized features of the circuit board to the inner filter structure. This further enables the additional assembly required to construct a tunable filter, inclusive of control circuitry. This invention enables the construction of high performance low cost fixed or tunable filters.
Tunable microstrip combline filters can be designed on the surface layers of a printed circuit board and the tuning reactance can be attached to the resonators using copper traces on the circuit board. Planar combline filters have also been embedded into inner layers of circuit boards using stripline technology. This allows the tuning elements to be soldered to the top layer of the circuit board and connected to the resonators by via holes. These types of filters can be fixed or tunable, but can be very lossy because the filter resonators are surrounded by lossy dielectric material.
Research and development of Substrate Integrated Waveguide (SIW) filters has been conducted over the past several years to address the problems mentioned above. Waveguide filters are high performance filters that can be tunable, but they are costly and large in size. SIW filters were developed to implement waveguide filters within a printed circuit board substrate. The ultimate goal of this effort was to reduce the size and cost of the filters. These filters have achieved their goals of smaller size and lower cost, but they suffer from higher loss because of the lossy dielectric material of the circuit board. However, SIW filters cannot achieve size reduction without the use of dielectric material. The electrical properties of the dielectric material reduce the wavelengths of the electro-magnetic waves within the material. But because the dielectric material is lossy, the size reduction that is achieved comes at a cost, which is additional signal loss in the dielectric material.
Accordingly, a need exists for a filter that functions well at microwave frequencies, can achieve narrow bandwidths, can achieve low signal loss and, if so needed, can be tunable. A need also exists for such a filter that can be constructed with reduced size and at reduced costs. Yet a further need exists for such a filter that can be easily and efficiently assembled without difficult structuring and soldering.